A variety of technologies are available for supporting high data rate communication. In applications where it is desired to support very high bandwidth it is common to provide an optical waveguide, also referred to as a fiber optic cable or optical fiber, between a light source and a receiver. Thus, a light source provides a light signal to an optical waveguide and the optical waveguide guides the light signal to the receiver. Unfortunately, in many cases it is not convenient or even feasible to string an optical fiber from one location to another location. In addition, optical waveguides are subject to physical damage.
An alternative communication system uses radio signals to transfer data. A radio communications system is much easier to install than a system that relies upon optical waveguides because typically there is no need for any physical infrastructure between the transmitter and the receiver. Unfortunately, radio communication systems offer lower bandwidth than optical communication systems. In addition, since radio signals typically are not well confined, the transmitted signal is fairly easy to monitor.
In order to provide high bandwidth communication between two locations that are fairly close together it is also known to use free space optics. A free space optical communication system uses typically a laser to emit an optical signal toward a receiver. The optical signal is not confined by an optical waveguide and so it is a requirement that the emitter and the receiver have an unobstructed line of sight therebetween. Free space optical (FSO) systems also have a few inherent drawbacks. For example, any object that is disposed between the signal emitter and the signal receiver will affect the propagation of the optical signals. In addition, the distance between the signal emitter and the receiver is limited to about 2 kilometers using conventional laser-based FSO equipment. Another challenge to using free space optical systems is that it is often difficult to ensure that the separate components are aligned correctly. More specifically, while the components may be properly aligned at one point in time, that alignment is subject to change under normal operating conditions. For example, if a laser-based emitter is disposed on the top of a very high tower then the laser may move slightly in very windy conditions. Alternatively, a train passing nearby may shake the earth beneath the tower enough that the laser is pointing momentarily in a direction that prevents good communications. Similarly, the receiver is also subject to small movements that may reduce signal quality.
The prior art teaches a variety of techniques for minimizing the effects of these types of alignment problems in FSO systems. For example, CANON™ supports a product called “CANOBEAM™” which is a laser signal emitter featuring a system to compensate for small, angular misalignments.
In U.S. Pat. No. 6,347,001 issued to Arnold et al., the entire contents of which is incorporated herein by reference, an error signal indicative of angular misalignment is generated. This signal is provided to an actuator that serves to compensate for the misalignment. The tracking system of Arnold et al. uses an actuated mirror to support both a communications laser and a fine tracking centroider. The actuator for controlling the orientation of the mirror relies upon an error signal that is proportional to an angular difference between the bore sight of the receiving terminal and the line of sight to the opposite transceiver.
The above-mentioned systems and techniques are suitable for their intended purpose of compensating for small, angular misalignments between emitters and sensors that are mounted to stationary structures. Unfortunately, this type of compensation does not allow for the use of FSO communication between mobile platforms, such as for instance moving vehicles, etc.
It would be beneficial to provide a system that supports robust free space optical communication by ensuring that the laser and receiver are properly aligned.